Positive Volcanic Clouds

There are several reasons why a volcanic cloud may be characterized by
a positive, rather than negative, band 4-5 brightness temperature
difference. The most common reason is that the cloud is still
opaque. The temperature difference
method relies upon radiation from the surface beneath the cloud passing
through the cloud, and being scattered and absorbed by particles in the
cloud. If it is opaque to thermal radiation, the signal seen by the
satellite comes from emission from the cloud top. At the temperatures
of typical clouds, this results in a positive temperature difference.
If the cloud is positive due to its opacity, one would expect that
it would become negative over time, and thus be able to be
discriminated using the band 4-5 temperature difference.

Examples

Mt. Spurr, Alaska: August, 1992
The best example the transition from an opaque volcanic to a transparent one.
The evolution of the volcanic cloud signal can be seen in a series of
images of the August 19, 1992 volcanic cloud. Volcanic clouds imaged
during eruption and shortly thereafter are optically thick and probably
contain abundant water droplets and/or ice, which makes their spectral
signal very much like a meteorological cloud. As the volcanic cloud
disperses and becomes transparent, the spectral properties of the cloud
change, first at the edge and then throughout. This produces a volcanic
cloud signal that can be distinguished using a brightness temperature
difference determined from thermal bands 4 and 5 of the AVHRR.

Sequence of Band 4 images

Sequence of Corresponding Band 4 minus 5 images

Redoubt Volcano, 1989-90
Typical examples of opaque volcanic clouds with a positive band 4-5 temperature
difference. Note that the negative values over land may be due to a
temperature inversion.

February 15, 1990 1316Z
Images: left band 4, right band 4-5

March 23, 1990 1331Z
Images: left band 4, right band 4-5

Augustine Volcano, 1986

Typical examples of opaque volcanic clouds with a positive band 4-5 temperature
difference. Note that the negative values over land may be due to a
temperature inversion.